1. Field of the Invention
The present invention relates to a display control apparatus of a display panel and a display device with the display control apparatus. Particularly the present invention relates to a display control apparatus which prevents distortion of a display screen when, for example, switching the video modes, and to a display device having the display control apparatus.
2. Description of the Related Art
A display panel device to which the present invention is applied is a device such as a plasma display panel (“PDP” hereinafter) or a liquid crystal display panel (“LCD” hereinafter), which generates display data by performing predetermined signal processing on an input video signal to display an image corresponding to the input video signal by means of the display data.
The above display panel device is configured with a display control apparatus which inputs, for example, an NTSC input video signal and a synchronizing signal (or a composite signal in which the both signals are combined) and processes the input video signal according to the synchronizing signal to create display data required for display control, and a display panel which is drive-controlled on the basis of the display data generated by the display control apparatus. The display control apparatus monitors a cycle of input synchronizing signal to judge a video mode, and generates the display data from the input video signal on the basis of the judged video mode.
To describe the PDP as an example, one frame is configured with a plurality of sub-frames having sustained discharge periods which are different from one another. By combining the plurality of sub-frames accordingly, luminance which is required in the frames is realized. For this reason, the display control apparatus extracts a gradation value of each pixel, from an input video signal supplied sequentially for each dot, on the basis of a synchronizing signal, and further converts the gradation value of each pixel into sub-frame data to generate the display data. In other words, the display data comprises the sub-frame data.
The sub-frame data has at least an address period and sustained discharge period, thus requiring corresponding periods. For this reason, the number of sub-frames which can be displayed within one frame depends on the image mode in which a cycle of the input synchronizing signal is different. Therefore, the display control apparatus needs to determine the video mode according to the input synchronizing signal, and generates the display data in accordance with the number of sub-frame corresponding to the determined video mode. Japanese Patent Application Laid-Open No. H8-76716, for example, describes internal display control performed in accordance with the different video modes as described above.
On the other hand, after generating the display data of a frame (this is the sub-frame data in the example of the PDP), the display control apparatus stores the display data in a frame memory once. In a following frame period, the display control apparatus reads the display data stored in the frame memory, and supplies it to the display panel. In the display panel, electrode drive is performed inside the panel in accordance with the supplied display data. This frame memory has a first frame memory region in which display data corresponding to an input video signal of a present frame is written, and a second frame memory region in which display data to be displayed in the present frame is read out. When the display data corresponding to the input video signal is written in the first frame memory region during a certain frame period, the display data is read out from the first frame memory region in the next frame period, and is used for display control in the display panel. Then, in the above frame period the display data corresponding to the input video signal is written in the second frame memory region. In this manner, the frame memory has a double buffer structure comprising the writing region and the reading region. Japanese Patent Application Laid-Open No. H10-307562, for example, describes such frame memory having the double buffer structure.
There are a plurality of types of video modes for the video signals of the display device. For example, in a NTSC TV signal, a video signal is configured on the basis of a 60 Hz-synchronizing signal, but a video mode with a 50 Hz synchronizing signal or a video mode with a 70 Hz synchronizing signal can be considered. It is required that the display device appropriately display images corresponding to the input video signals of such different video modes.
A problem in this case rises in the display control method when the video modes are switched. When a video mode with a 60 Hz synchronizing signal is switched to a video mode with a 50 Hz synchronizing signal, the display control apparatus monitors the cycle of the input synchronizing signal, judges that the cycle of the input synchronizing signal has been switched, and generates display data on the basis of the judged video mode.
However, at the moment when the video mode is switched, in the frame in which the switching is carried out, display data, which is created from an input video signal of a video mode before the switching, and display data, which is created from an input video signal of a video mode after the switching, are written in the frame memory. Therefore, when the written display data is read out and supplied to the display panel in the next frame, a distorted image is displayed.
Similarly, when a transmittance station (channel) is switched, synchronization is not performed between the channels even if the same video mode is used. Therefore, at the time of switching, display data of both channels are written in a frame memory in the frame in which the switching is generated, thus the same problem, which has occurred when switching the video mode, occurs.
Moreover, when judging whether to switching a synchronizing signal, the display control apparatus detects switching of a video mode only after confirming that a synchronizing signal of the same cycle is inputted continuously throughout a plurality of frames. Accordingly, it can be avoided that switching of the video mode is detected mistakenly by overresponding to noise that overlaps with the synchronizing signal. Therefore, in several frame periods immediately after the video mode is switched, the video mode in the display control apparatus and the video mode of the input video signal do not conform to each other, thus the displayed image during this period is distorted.
In the prior art, in consideration of the above problems, a whole black image is displayed in a predetermined number of frames when switching the video mode. However, displaying the whole black image in the frames is not always tolerated by users, thus other display control apparatus is desired.